LONDON (Reuters) - A nerve agent was used to poison former Russian double agent Sergei Skripal and his daughter Yulia in the southern English city of Salisbury, Britain’s top counter-terrorism officer said.

What is known so far?

WHAT HAPPENED?

At 4:15 p.m. (1615 GMT) on March 4, Wiltshire police got a call from a member of the public about two people who were acting strangely.

Police found Skripal, 66, and Yulia, 33, unconscious on a bench outside The Maltings shopping centre in Salisbury.

The pair had no visible injuries and were taken to Salisbury District Hospital, where they are in critical condition.

CCTV footage carried by British media showed a woman and a older man walking near the bench where Skripal was found. It was not clear who they are or whether they are linked to the case.

A police officer who responded to the incident is in hospital. He is now able to talk though his condition remains serious, interior minister Amber Rudd said on Thursday.

Counter-terrorism police took control of the investigation on March 6. Rudd said Britain would work out who was behind the attack and respond robustly.

WHAT POISONED THEM?

Britain’s top counter-terrorism officer, Mark Rowley, said Skripal had been exposed to a nerve agent. The agent had been identified but he declined to give any more details about it.

“This is being treated as a major incident involving attempted murder by administration of a nerve agent,” Rowley said. “I can also confirm that we believe the two people originally who became unwell were targeted specifically.”

There was no evidence of a wider threat to public safety, he said.

Samples from the scene are being tested at Porton Down, Britain’s military research laboratory. In Salisbury, some police investigators wore chemical and biological protection suits.

WHAT IS A NERVE AGENT?

Nerve agents are small molecules based on phosphorus that interfere with nerve transmission, said Andrea Sella, professor of inorganic chemistry at University College London.

“In essence what they do is to block the mechanism that allows a nerve to reset itself after a signal has been transmitted,” he said. “This causes a pretty systemic collapse of many bodily functions.”

WHERE DID THE POISONING HAPPEN?

Skripal and his daughter were in Salisbury city centre from around 1:30 p.m. on March 4. Police sealed off the Zizzi restaurant on Castle Street, The Bishop’s Mill pub in The Maltings and a business park near the town of Amesbury.

“We would like to hear from anybody who visited the area close to The Maltings shopping centre where these two people were taken ill on Sunday afternoon, and may have seen something that could assist the investigation,” Rowley said.

WHO IS SERGEI SKRIPAL?

Sergei Viktrovich Skripal was born in the Soviet Union on June 23, 1951.

Little is known about his early life but he served in the GRU military intelligence service. He reached the rank of colonel in the GRU but left in 1999 to work in the Russian foreign ministry. He also taught at the Russian Defence Ministry’s defence academy in Moscow.

Skripal was arrested in December 2004 by Federal Security Service agents on suspicion of treason: passing secrets to Britain’s MI6 intelligence agency. Skripal had been “turned” by MI6 in 1995, the FSB said, and had been in contact with British agents at the Moscow embassy.

A police officer guards Zizzi's restaurant where Sergei Skripal and his daughter Yulia are known to have visited shortly before they were found in the centre of Salisbury, Britain, March 8, 2018. REUTERS/Peter Nicholls

In a secret trial at a Moscow military court, he confessed to his treachery and to selling the names, addresses and codenames of several dozen Russian agents to MI6, Russian media said when his conviction was announced in 2006.

Many of the Russian agents he betrayed were so-called deep cover spies in Britain and Europe. Russian media said his motivation was financial: he received more than $100,000 paid into a Spanish bank account.

Moscow’s respected Kommersant newspaper reported in 2006 that the FSB considered that the damage he had done to Russian spying operations was comparable to that caused by Oleg Penkovsky, another GRU colonel.

Penkovsky, a friend of the then GRU chief, informed British and American spies of a Moscow operation to place nuclear missiles in Cuba. The scandal led to the 1962 Cuban crisis and the world on the brink of nuclear war for several days.

Penkovsky was arrested in 1962 and executed in 1963 after being found guilty of high treason and espionage.

Skripal, who was shown wearing a track suit in a cage in court during his sentencing, was jailed for 13 years in 2006.

“The spy caused significant damage to Russia’s ability to defend itself and to state security,” the FSB said at the time, Itar-Tass reported.

WHY IS SKRIPAL IN BRITAIN?

In June 2010, the U.S. Federal Bureau of Investigation said it had broken up an undercover Russian spy ring in the United States.

Dubbed the “illegals”, some of the Russians had spent years quietly collecting information and trying to meet Americans with political ties.

Skripal was pardoned by then-President Dmitry Medvedev and put on a plane for Vienna, where in July 2010 he was exchanged, along with three others, for the 10 Russian spies caught in the United States.

The spy swap, one of the biggest since the Cold War, took place on the tarmac of Vienna airport where a Russian and a U.S. jet parked side by side before the agents were exchanged.

One of the Russian spies exchanged for Skripal was Anna Chapman, who was feted as a hero by Moscow on her return.

WHY WAS HE IN SALISBURY?

Since emerging from the John le Carre world of high espionage and betrayal, Skripal lived modestly in Salisbury and kept out of the spotlight until he was found unconscious on Sunday.

His house in Salisbury was bought for 260,000 pounds ($360,000) in 2011. Skripal was listed at living there under his own name. In the years since he found refuge in Britain, Skripal lost both a wife and son.

His son Alexander died on July 18, 2017 aged 43 in St Petersburg, British media reported. The details are unclear.

His wife Liudmila died on Oct. 23, 2012 of cancer at the age of 60, according to British media. Reuters has not confirmed the exact cause of death of either.

WHAT IS GRU?

Russian military intelligence service is known by its Russian acronym GRU, which stands for Main Intelligence Directorate. Moscow’s other, better-known Foreign Intelligence Service (SVR) is the successor to the KGB’s First Chief Directorate.

Unlike the KGB, GRU was not split up when the Soviet Union collapsed. It has a special status and answers directly to the chief of the general staff, one of three people who control Russia’s portable nuclear trigger. GRU chiefs are picked by the president.

GRU has agents across the globe. It also has special forces units that fought in many post-World War Two conflicts including Afghanistan and Chechnya.

GRU, whose emblem features a bat hovering above the globe, was founded as the Registration Directorate in 1918 after the Bolshevik Revolution. Revolutionary leader Vladimir Lenin insisted on its independence from other secret services, which see GRU as a rival.

The public was given a rare chance to see parts of GRU’s Moscow headquarters when President Vladimir Putin visited it in 2006. He was shown taking part in shooting practice.